FMRI measures brain activity through a secondary effect of neuronal activation, the BOLD effect, which comprises contributions from metabolism, blood flow, and blood volume, and which is not quantitatively understood in terms of these parameters. The time course of the BOLD signal is also somewhat variable from subject to subject, which tends to decrease statistical power of the experiment if the variability is random, and can lead to systematic errors if the variability is systematic. We have performed a pilot study of 28 children and adults, where we measured the shape of the hemodynamic response in a visual task. Unexpectedly, we found a systematic variation of the width of this function with age. We would now like to investigate this effect further. Specifically, we would like to determine the physiological origin of the systematic variability. To this end, we are planning to measure BOLD, blood flow, and blood volume in three groups of subjects (children, young adults, older adults) in response to a visual stimulation task, and to compare the results to known physiological differences between different age groups. Also, we would like to determine if an age-specific BOLD model function should generally be used to increase statistical power and to avoid bias in experiments where different age groups are compared. [unreadable] [unreadable]